The present invention relates to an exhaust gas cleaner and a method of cleaning an exhaust gas by using such an exhaust gas cleaner, and more particularly to an exhaust gas cleaner capable of efficiently removing nitrogen oxides and fine carbonaceous particles simultaneously from exhaust gases of diesel engines, etc., and a method of cleaning an exhaust gas by using such an exhaust gas cleaner.
Recently, fine carbonaceous particles (hereinafter referred to simply as "particulate matter") and nitrogen oxides (hereinafter referred to simply as "NOx") contained in exhaust gases of diesel engines, etc. are causing environmental problems. In particular, the particulate matter having an average particle size of 0.1-1 .mu.m is likely to float in the air and be inhaled by breathing. As a result of recent clinical tests, it is confirmed that the particulate matter contains carcinogenic substances.
As a method for removing the particulate matter, there are two candidates:
One method comprises the steps of trapping the particulate matter in heat-resistant filters by filtrating exhaust gases, and burning the trapped particulate matter by a burner, an electrical heater, etc. when a pressure drop increases due to the particulate matter accumulated, to regenerate filters. The heat resistant filters may be honeycomb-type ceramic filters, foam-type ceramic filters having three-dimensional network structures, steel wool, wire mesh, etc. The other method comprises the step of trapping and self-burning the particulate matter by the action of catalysts supported by the above filters.
In the former method, as the efficiency for removing the particulate matter increases, the pressure drop increases more quickly, meaning that the filters are required to be regenerated more frequently with a high reliability, leading to an economical disadvantage.
In contrast, the latter method is considered to be excellent as long as a proper catalyst exists, which is capable of maintaining a catalytic activity under the conditions of the exhaust gases of diesel engines with which the catalyst is brought into contact (gas composition, gas temperature, etc).
However, since a diesel oil is used as a fuel in diesel engines, exhaust gases contain a large amount of SO.sub.2. The oxygen concentration in exhaust gases varies in a wide range of 2-20%, depending upon the operation conditions of diesel engines. Under these conditions, there has been no established method of well igniting and burning fine carbon particles accumulated without causing secondary pollution.
For instance, as catalysts for removing particulate matter from exhaust gases, which have been proposed so far, there are precious metal catalysts and base metal catalysts. The precious metal catalysts are durable and function efficiently to oxidize CO and unburned hydrocarbons (hereinafter referred to simply as "HC"), but it is likely to convert SO.sub.2 existing in exhaust gases to SO.sub.3, leading to secondary pollution. Besides, there are problems such that ignition activity of soot in the particulate matter is lowered. On the other hand, the base metal catalysts are effective for removing the particulate matter, but their durability is poor.
Most of the catalysts for exhaust gas cleaners, which have been proposed hitherto, mainly function to lower the ignition temperature of particulate matter, leaving unsolved the problems of removing NOx from exhaust gases of diesel engines having a large oxygen concentration and/or a considerably variable oxygen concentration.
Japanese Patent Laid-Open No. 3-47539 discloses an exhaust gas cleaner comprising a heat-resistant, porous filter, a first catalyst supported by the filter in the inlet region, and a second catalyst supported by the filter in the outlet region, the first catalyst consisting essentially of (a) one or more alkali metal elements, (b) one or more elements in Groups IB, IIA, IIB, transition metal elements of the Periodic Table and Sn, and (c) one or more rare earth elements, and the second catalyst consisting essentially of at least one platinum-group element. By this exhaust gas cleaner, NOx and particulate matter are removed by the first catalyst in the inlet region by using mainly particulate matter and HC as reducing agents, and HC, CO, and other toxic gases are removed by the second catalyst in the outlet region. However, according to their research, the catalytic activity of this catalyst is not satisfactory even when any one of the transition metal elements is selected.